Pre- and postflight systolic time intervals during LBNP: the second manned Skylab mission

After space flight of 59 d, Skylab 3 astronauts were stressed with lower body negative pressure (LBNP). During this stress procedure vectorcardiograms, pneumograms, phonocardiograms, and carotid pulse tracings were monitored and recorded onto analog tape. Accepted techniques were used to measure the intervals of systole. The postflight results were compared to multiple preflight tests and each of the three crewmen served as his own control. Immediately postflight, there were elevations in heart rate and blood pressure in response to a fixed level (-50 mm Hg) of LBNP. Total electromechanical systole, (Q-S2) I, was unchanged. Ejection time index (ETI) was depressed at rest and during stress, while pre-ejection period was elevated compared with preflight values. Systolic time intervals (STI) were within preflight limits after 1 month on earth in all crewmen. Resting STI returned sooner than did stressed STI. The magnitude and direction of STI in the postflight period were similar to those obtained from patients with moderate heart disease, although signs and symptoms were absent in the astronauts. However, the abnormality of the stressed STI persisted after both blood volume repletion and lowered afterload. These findings suggest a compromise in cardiac function, peripheral circulatory integrity, or both after exposure to long-duration space flight, and are consistent with findings reported after 3 weeks of absolute bedrest.